In the past, as the method of attaching a device inside of a pipe to uniformly mix fluid flowing through the inside of the pipe, as shown in FIG. 18, use of a twisted blade type static mixer element 151 has been the general practice (see, for example, Patent Literature 1). Usually, the static mixer element 151 is comprised of a square plate twisted 180 degrees about its longitudinal axis as a minimum unit member and has a plurality of such minimum unit members integrally connected in series so that the twisting directions become mutually different directions. This static mixer element 151 is arranged in a pipe 152, male connectors 153 are attached to the two ends of the pipe 152, flare nuts 155 are attached, and fastening nuts 154 are fastened, whereby a static mixer is formed. At this time, the outside diameter of the static mixer element 151 is designed to be substantially equal to the inside diameter of the pipe 152, so the fluid is able to be effectively agitated.
However, the method of mixing fluid using this conventional static mixer is to agitate a flow of fluid along the flow, so as shown in FIG. 19a. Therefore, although it is possible to make the distribution of concentration in the radial direction of the pipe uniform without any unevenness, it is not possible to make the distribution of concentration in the axial direction (flow direction) uniform without any unevenness, as shown in FIG. 19b. For this reason, for example, when mixing water and a chemical at the upstream side of the static mixer, if the mixing ratio of the chemical temporarily increases, the fluid will pass through the static mixer in a state partially denser in concentration in the flow path. At this time, even if the water and chemical are agitated and made uniform in concentration in the radial direction, in the axial direction (flow direction), locations in the flow path where the concentration partially becomes denser will end up flowing to the downstream side in the dense state as they are without being diluted much at all (see FIG. 19b). Due to this, when connected to a semiconductor washing apparatus, in particular, an apparatus which directly coats the surface of a semiconductor wafer with a chemical to perform various types of treatment, there is the problem that different concentrations of the chemical are coated on the surface of the semiconductor wafer and thereby causes defects.
As a method for avoiding unevenness in the distribution of concentration in the axial direction (flow direction), the method of installing a tank in the middle of the flow path, storing fluid temporarily in the tank, making the concentration in the tank uniform, then running the fluid (not shown) etc. may be mentioned. However, there is the problem that a large space is required for installing the tank and therefore the apparatus becomes larger. Further, the transport of the fluid from the tank again requires a pump, piping, etc., so the number of the parts for use increase, and cost is incurred for installing the pipeline. Further, with this method, the fluid stagnates in the tank, so becomes a cause of proliferation of bacteria. The bacteria proliferating in the tank flows into the pipeline and, in a semiconductor production line, the bacteria deposits on the semiconductor wafer and therefore causes defects.